1. Field of the Invention
This invention relates to a system for gravity filling of liquid storage tanks, and in particular to a liquid storage tank system incorporating an automatic shut-off mechanism which prevents overfilling the storage tank.
2. Description of the Prior Art
Considerable difficulty has been experienced in the past in attempting to prevent the overfilling of underground storage tanks at gasoline service stations. Overfilling and the resulting spillage has been a common occurrence during the gravity filling of underground storage tanks. This spillage is wasteful, hazardous, and causes environmental pollution.
Numerous attempts have been made to overcome this difficulty. However, in most of the prior structures which have been proposed, extensive modification of existing equipment has been required.
For a number of years, an automatic shut-off nozzle has been used in gasoline service stations for automatically shutting off the supply of fuel from a gasoline or other petroleum fuel pump to a vehicle when the level of liquid in the vehicle tank rises above a predetermined level. This type of nozzle is described in U.S. Pat. No. 3,196,908. In this device, a vacuum pressure is established by means of a venturi section within the nozzle. In this apparatus, however, the liquid passes through the nozzle under the pressure of the delivery pump so that no difficulty is experienced in drawing a sufficient vacuum at the venturi to activate the pressure-sensitive latching mechanism to release the closure valve when the level of liquid rises above the end of the vent line which is located within the vehicle storage tank. Although it is known that this device has operated successfully for many years, it has not been possible to employ this system when gravity filling a liquid storage tank, such as an underground storage tank, because of the velocities involved.
An attempt to incorporate the principles of the automatic shut-off nozzle into a system for gravity filling underground storage tanks was proposed in U.S. Pat. No. 4,040,455, issued to Swain et al. Swain et al. discovered that the line extending from the tank truck of the vehicle to the underground storage tank was a siphon at which at certain locations there was a significant negative pressure to draw a partial vacuum sufficient to activate the latching mechanism. This partial vacuum was utilized by extending a vent line down through the fill line, with one end of the vent line opening into the underground storage tank at a predetermined level within the tank, and the other end of the vent line communicating with a pressure-sensitive latching mechanism. A vacuum passage extended between the vent line and the fill line in order to draw a partial vacuum in the vent line. The partial vacuum would normally be vented by drawing air from within the tank until the level of liquid in the tank rose above the bottom end of the vent line, whereupon the partial vacuum would be drawn on the pressure-sensitive latching mechanism to release the latching mechanism, causing the shut-off valve to move to the closed position and interrupting the flow of liquid into the storage tank.
A major disadvantage of the system shown in the patent to Swain et al. was that it required the automatic shut-off mechanism to be installed at the end of the fill line extending from the tank truck. Thus, it was the responsibility of the tank truck operator to install the automatic shut-off mechanism on his fill line. The major advantages of automatic shut-off, however, did not primarily benefit the tank truck operator but, instead, benefited the service station operator. It was primarily the service station owner or operator who desired to prevent overflows and the resulting spills, since the spills occurred on his property, and he was primarily responsible for cleaning them up. Thus, the service station operator had a financial incentive for installing an automatic shut-off mechanism. The tank truck operator did not.
The automatic shut-off mechanism disclosed in the patent to Swain et al. had to be installed in the fill line from the truck because it relied upon the pressure drop within the fill line from one point in the tank truck fill line to the underground storage tank in order to draw a partial vacuum sufficient to activate the latching mechanism. Thus, it was not possible to install the automatic shut-off mechanism of Swain et al. as part of the underground storage tank fill structure, since the pressure drop between the tank truck fill line, which was located above ground level, and the top of the underground storage tank was essential to the operation of the mechanism of Swain et al. Yet, it is highly desirable to locate the automatic shut-off mechanism entirely with the installation at the service station rather than in the tank truck fill line. If the automatic shut-off mechanism were installed entirely on site at the service station, it would be necessary to design the mechanism so that it could be installed entirely underground, but the design of Swain et al. is unsuitable, since it cannot be installed underground.
It would also be advantageous to install such a mechanism in a containment manhole in the event that fuel or other product is spilled during the filling process. A containment manhole reduces the pollution of ground water and product spillage which may occur during the filling of the underground tank by capturing the overspill and holding it in the containment manhole. The contained product may then be removed from the manhole, or may be drained directly into the underground tank.
Examples of such containment manholes are shown in U.S. Pat. Nos. 3,633,219, issued to Byrd; 4,278,115 issued to Briles et al.; and 4,527,708, issued to Bundas et al. The Briles et al. and Bundas et al. patents show containment manholes each having a flexible seal around the riser pipe at the location at which the riser pipe enters the manhole. The Briles et al. patent also discloses a drain within the manhole through which the spilled product may be removed from the manhole. While Briles et al. disclose draining the product to a separate holding tank, the Byrd patent discloses connecting an exterior drain to the riser pipe so that spilled material may be drained directly into the riser. While these designs provide certain advantages, there are various difficulties resulting from the design of these manholes.
One problem relates to the drainage of the spilled product in the manhole from the manhole into the underground tank. Prior art designs have typically used an external hose connecting the drain in the manhole to a fitting on the riser. A flexible hose has typically been used. However, this hose is susceptible to external damage and internal blockage. Blockage could occur due to debris contained in the manhole which would be drained into the drain hose and block the drain. Damage could result during installation, or due to frost heave or settling, possibly resulting in a leak to the hose or resulting in the hose breaking or becoming disconnected from the riser. Because the manhole is externally fitted and back-filled, it is extremely difficult to remove the manhole to repair the drain hose without excavation of the entire site.
Another problem with the prior art containment manholes related to the use of the elastomeric seal to achieve integrity between the riser pipe and the opening in the manhole through which the riser extended. When the manhole moved axially due to frost heave or settling, the movement could introduce debris into the seal gland and cause damage to the seal or to the surface which the seal must act upon. If either the seal or the surface were damaged, the manhole would no longer contain liquid, since liquid could leak through the damaged seal around the riser pipe.
Another problem also related to the types of seals employed in the prior art manholes. The seals used in the prior art manholes required rather precise glands to hold the seal in a particular relationship to the seal surface. This requirement did not allow for radial movement of the manhole. The manhole might move radially due to tank settling, pavement settling, thermal expansion of the surrounding pavement, or seismic shock. Such radial movement could cause the seal gland to bind and damage the seal or the surface against which the seal acts. Furthermore, radial movement of the manhole could result in the opening in the manhole binding against the riser pipe, resulting in severe stresses transmitted through the riser pipe which could cause damage to the pipe or to the underground tank.